fix use of `binding' data abstraction

fix use of `binding' data abstraction

* module/system/vm/assemble.scm (make-temp-binding, btemp:name)
  (btemp:extp, btemp:index): Don't abuse program.scm's make-binding to
  make something that actually isn't a binding.
  (codegen): Do use program.scm's make-binding to make something that
  actually is a binding.

* module/system/vm/program.scm (binding:start, binding:end): New
  accessors.
  (make-binding): Expand to have the start and end arguments in the
  constructor.